1. Field
This disclosure relates to adjustment of colors in color reproduction devices, more particularly to processing of user-designated color adjustments.
2. Background
Color reproduction devices take a colored original and convert it into image data that is then reproduced on another substrate. These devices may include stand-alone copiers and fax machines, and multi-function peripherals (MFP) that combine these functions into one device. The printing substrate is typically paper, but can include photographic paper, transparencies, etc.
Some of these devices allow the user to adjust the colors of the original through a front control panel. Similar to increasing or decreasing the lightness in a black and white original, the user may choose to boost or reduce colors. However, this may cause problems in the color image processing that occur after the original is scanned.
In a typical color reproduction device, the original image is scanned and converted to red-green-blue (RGB) data. This data is then converted to luminance/chrominance color space, referred to as LAB, LCC or opponent color space. In an 8-bit system with values between 0 and 255, neutral colors are typically represented with the value 128 on the two opponent color axes. After adjustment by the user, chromaticity values may exceed thresholds or trigger values established in unadjusted cases of the color image processes.
For example, many color reproduction devices perform background detection and suppression in which background colors may be removed. This is common when the original image resides on a colored piece of paper. The background color is removed to prevent rendering of the background in the printer, which wastes resources and takes a longer amount of time. This process often uses a threshold that allows near-neutral colors to be removed from the image. After the user adjusts the colors, the value shifts may cause the background to be rendered in the copy.
Another problem may occur if the user makes a large gain adjustment. The values of colors already near a maximum value become limit-clipped. This results in loss of chromatic gradations. Subsequent operations cannot restore these chromatic gradations after they have been clipped. These images are rendered as uniform or nearly uniform, degrading the quality of the resulting reproduction.
Therefore, it would seem useful to have a method of adjusting user-designated color inputs to avoid interfering with further color image processing.